The present invention relates to keratographic instruments used for measurement of the cornea of an eye. More specifically, the present invention relates to such instruments having an illumination arrangement to reduce alignment sensitivity.
A keratometer measures the curvature of the cornea at one or several radial positions with respect to an eye axis defined by eye orientation. The eye axis is usually close to the visual axis. The keratometer provides a keratograph showing the curvature of the cornea at many points, the points being densely distributed over a moderate to large fraction of the cornea.
There are several types of keratographs in use or proposed for use. Most of these have a light source in the form of a set of rings with progressively greater diameters, the rings being concentric about a common axis. The image of the corneal reflection of these rings is observed by a camera and analyzed to determine corneal shape at multiple points.
Some modern versions of the keratometer use a TV camera to view the image of the rings of light in the subject's eye and digitize the image for computer analysis of the cornea shape. Unfortunately, errors are introduced into this information if the cornea is displaced from the position assumed in the calculations. That is, the image of a light which has been reflected by a cornea has a pattern which depends upon the contour of the cornea and the position of the cornea. To the extent that the cornea position may be different from that assumed by the calculations, an instrument will introduce an inaccuracy in the calculations of the cornea shape. Since the inaccuracy in cornea shape due to slight displacement of the cornea from an assumed position can be shown geometrically to increase as the light source gets closer to the eye, one would want the light source to be relatively far away from the cornea of the eye. However, moving the light source far away from the observed eye will restrict the field of view of the cornea of the eye because the light of some rings will be interrupted by the nose and brows of the person.
Among prior patents for cornea curvature measurements, U.S. Pat. No. 4,662,730 issued May 5, 1987 to Outwater shows a scanning keratometer using a holographic element functioning as a lens to scan a beam of light over the cornea, and to descan the light scattered back from the beam by the cornea.